With the recent rapid increase in the demand for coated papers, the processing speed at paper coating has been increased, and the increase in the production of printed matter has expedited speeding up of printing. In offset printing, in particular, this tendency is becoming more and more pronounced.
Therefore, a copolymer latex to be used as a binder which is one of the components of paper coating compositions is required to have the following properties.
First, it should afford good adhesive strength. When the adhesive strength is low, the mechanical force applied to the coated paper surface in the step of printing will cause the pigment to drop off and the coated layer to peel off from the base paper. The higher the printing speed is and the greater the number of overlapping printings is, the severer such paper surface destruction is. Therefore, a binder excellent in adhesion among pigment particles and between the coated layer and the base paper is required.
Second, it should have good water resistance. In particular, in offset printing where "damping water" is used, the coated paper surface is required to be resistant, in a wet state, to the mechanical force in printing or, in other words, have water resistance.
Further, with the speeding up of printing, it is required to cause much better ink acceptability as compared with the conventional binders.
In addition, coated papers are required to have not only the above properties but also optical characteristics such as sheet gloss.
While, as mentioned above, coated papers are required to have such characteristics as adhesion strength, water resistance, ink acceptability and sheet gloss, none of the prior art coated papers has these characteristics balanced at high levels. This is because the adhesive strength, water resistance, ink acceptability and sheet gloss are in contradictory relations with one another.
Thus, while it is known that the water resistance can be improved by adjusting the gel content of the polymer latex to be used as a binder to a relatively low level, a decrease in gel content results in decreased adhesive strength and decreased ink acceptability and a further decrease in said gel content leads to decreased water resistance. It is known that the ink acceptability and sheet gloss can be improved by increasing the polymer latex particle size or increasing the glass transition temperature. However, either measure encounters a difficult technological problem, namely the adhesive strength and water resistance both decrease.